Nitric oxide (NO) and cytochrome P450 (CYP450)-mediated eicosanoids, especially 20-hydroxyeicosatetraenoic acid (20-HETE) are important determinants of a number of integrated body functions, including maintenance of vascular tone and renal function; hence, they are implicated in the genesis of hypertension. As the physiological actions of NO are attributable to the oxidation of iron, NO thereby modulates the activity of hemoproteins like the CYP450 enzyme system. The NO system is, therefore, an endogenous regulatory pathway for CYP450 eicosanoid production. NO also modulates the production/activity of other humoral factors, notably, endothelin (ET) peptides. Following inhibition of NO synthase (NOS), the ensuing renal hemodynamic changes which have been mainly attributable to withdrawal of NO may, therefore, reflect enhanced expression of vasoconstrictor CYP450 eicosanoids, especially 20-HETE and/or ET peptides. Based on: 1) reduced renal microsomal synthesis of HETEs by nitroprusside, an NO donor; 2) attenuation of the renal responses to Nw-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, by 12,12 dibromododec enoic acid (DBDD), an inhibitor of 20-HETE synthesis, and BMS182874, an ETA receptor antagonist; and 3) ET-1-induced renal release of 20-HETE, we hypothesize that NO inhibition of endogenous 20-HETE production, possibly linked to ET production, is an important homeostatic mechanism for maintenance of vascular tone and renal function. We further propose that 20-HETE is a counterregulator of NO activity. The Specific Aims of this proposal, therefore, are: 1) to determine the effect of NO on CYP450-dependent arachidonic acid (AA) metabolism and on the renal expression of CYP4A mRNA and protein; 2) to establish the contribution of 20-HETE to the renal hemodynamic and excretory responses induced by L-NAME; and 3) to determine if the involvement of 20-HETE in L-NAME response is linked to ET peptides. Conversion of AA to 20-HETE will be evaluated in renal microsomes incubated with NOdonors, and in microsomes from rats treated with L-arginine or L-NAME. Urinary production of 20-HETE and ET-1 will be determined in rats treated with the same agents. The mechanism of the reduced production of 20-HETE by NO will be evaluated by determining the expression of CYP4A mRNA and protein. Acute renal clearance studies will be performed to establish the contribution of 20-HETE in the renal functional response to administration of L-NAME. The role of 20-HETE in the dynamic relationship between vascular and tubular effects following NOS inhibition will also be examined in chronic studies in which hypertension will be induced with L-NAME. As ET-1, which we showed to stimulate 20-HETE production has also been shown to increase following L-NAME treatment, we will determine if 20-HETE production has also been shown to increase following L-NAME treatment, they will determine if 20-HETE production is coupled to ET production/activation of ET receptors and, if so, which receptors are involved. These studies should enhance our knowledge of the interplay between NO, 20-HETE and ET-1; endogenous regulators of renal function and systemic hemodynamics, and thus help our understanding of the pathophysiology of conditions characterized by inadequate production of NO.